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ORGANIZATION - Ownership Interests in Natural Gas Pipeline Systems (Details)
3 Months Ended 12 Months Ended
Jun. 01, 2017
Mar. 31, 2017
LimitedPartnership
Dec. 31, 2016
LimitedPartnership
mi
Jul. 31, 2017
Jun. 30, 2017
May 31, 2017
Apr. 28, 2017
Jan. 31, 2016
Jan. 01, 2016
Organization                  
Number of intermediate limited partnerships through which pipeline assets are owned | LimitedPartnership   3 3            
Northern Border                  
Organization                  
Remaining ownership interest (as a percent)     50.00%            
Interest acquired (as a percent)   50.00% 50.00% 50.00% 50.00% 50.00% 50.00%    
Portland Natural Gas Transmission System                  
Organization                  
Interest acquired (as a percent)               49.90%  
TransCanada | Portland Natural Gas Transmission System                  
Organization                  
Remaining noncontrolling ownership interest (as a percent)     38.29%            
Portland Natural Gas Transmission System                  
Organization                  
Interest acquired (as a percent) 11.81%               49.90%
GTN                  
Organization                  
Length of pipeline owned (in miles)     1,377            
Remaining ownership interest (as a percent)     100.00%            
Bison                  
Organization                  
Length of pipeline owned (in miles)     303            
Remaining ownership interest (as a percent)     100.00%            
North Baja Pipeline, LLC                  
Organization                  
Length of pipeline owned (in miles)     86            
Ownership interest (as a percent)     100.00%            
Tuscarora Gas Transmission Company                  
Organization                  
Length of pipeline owned (in miles)     305            
Ownership interest (as a percent)     100.00%            
Northern Border                  
Organization                  
Length of pipeline owned (in miles)     1,412            
Ownership interest (as a percent)     50.00%            
Portland Natural Gas Transmission System                  
Organization                  
Length of pipeline owned (in miles)     295            
Ownership interest (as a percent)     61.71%            
Great Lakes                  
Organization                  
Length of pipeline owned (in miles)     2,115            
Ownership interest (as a percent)     46.45%            
Great Lakes | TransCanada                  
Organization                  
Remaining noncontrolling ownership interest (as a percent)     53.55%            
Iroquois                  
Organization                  
Length of pipeline owned (in miles)     416            
Ownership interest (as a percent)     49.34%            
Remaining ownership interest (as a percent)     50.66%            
Iroquois | TransCanada                  
Organization                  
Remaining ownership interest (as a percent)     0.66%            
Iroquois | Dominion Midstream                  
Organization                  
Remaining ownership interest (as a percent)     25.93%            
Iroquois | Dominion Energy                  
Organization                  
Remaining ownership interest (as a percent)     24.07%            
ONEOK Partners, L.P. | Northern Border                  
Organization                  
Remaining ownership interest (as a percent)     50.00%            
Subsequent event | Portland Natural Gas Transmission System                  
Organization                  
Ownership interest (as a percent) 11.81%                
Subsequent event | Portland Natural Gas Transmission System                  
Organization                  
Ownership interest, including acquired interest (as a percent) 61.71%